Replace hand-rolled spatial canvas drag with Konva

The custom pointer-capture + frozen-camera drag math was fragile and
felt worse than before. Konva handles the drag gesture itself; the
"me" node's screen position stays pinned to the stage center via
dragBoundFunc + a worldLayer offset that recenters on every dragmove,
so the world pans under the avatar instead of the avatar drifting off
center. Grid, peers, and the audible-range circle all live in the same
world-space layer so they pan together.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -17,6 +17,7 @@
     "@opentelemetry/sdk-logs": "^0.222.0",
     "@opentelemetry/sdk-node": "^0.222.0",
     "@opentelemetry/semantic-conventions": "^1.43.0",
+    "konva": "^10.6.0",
     "matrix-js-sdk": "^42.4.0",
     "ws": "^8.18.0"
   },